Dagon (aka, the Prince of the Depths) is an obyrith, one of the demons that predated not only mortal life but the gods themselves. Originally, like all the other obyrith, it's form could drive a mortal insane. Over time, it evolved and took it's more recognizable shape. Which is a 40 ft hybird combining fish and octopus elements with black rubbery skin, many unblinking red eyes, and numerous tentacles. His maw is full of rows of long fangs and shielding his skin are fish scales.
Dagon is a demon lord of the Abyss, who rules the 89th layer. In the material plane, he and his minions lurk in the Sea of Fallen Stars and the waters that are west of Maztica. Infesting the dreams of those swimming the Sea of Fallen Stars with nightmares. Over millennia, he created five "wild tides' which swept great numbers of Sharlarin into the Sea of Fallen Stars, to be slaughtered by his minions. Eventually the shalarin abandoned their gods and turned to worshiping Dagon.